1. Field of the Invention
The present invention relates to a fluidized bed incinerator, and more particularly, to a fluidized bed incinerator and a combustion method in which generation of NOx, CO and dioxine can be suppressed at the same time.
2. Description of the Related Art
Exhaust gas such as NOx, CO, and dioxine are generally prescribed as regulation object materials about environmental quality. These materials can be decreased by providing a post processing apparatus to an incinerator. However, it is desirable from the viewpoint of the cost reduction in the manufacture, operation and maintenance of the incinerator to suppress the generation of these materials in the incinerator.
As one of the suppressing techniques of the NOx generation in combustion, a conventional technique is known in which air for the combustion is supplied to 2 steps. In the first step, an air surplus rate of supplied air is set to in a range of 0.8 to 0.9. In the second step, air is supplied to supplement a lack of air, resulting in complete combustion in the whole system. In this technique, the increase of flame temperature and the appearance of a local high temperature region are prevented by restraining rapid combustion reaction, and the generation of NOx is suppressed through the decrease of an oxygen quantity. In this technique, however, it is easy for incomplete combustion and unstable combustion to be caused, and they must be careful of the generation of non-combusted components such as CO. Therefore, this technique needs to be used together with another exhaust gas processing technique.
FIG. 1 is a diagram showing the structure of another conventional fluidized bed incinerator disclosed in Japanese Patent No. 2,637,449. The conventional fluidized bed incinerator will be described with reference to FIG. 1. The fluidized bed incinerator is composed of a combustion furnace 113, a cyclone 117, and a hopper 118. The combustion furnace 113 is composed of a first air supply port 101, a second air supply port 102, a furnace output port 105, a fuel input port 110, a heat transferring section 111, and a convectional heat transferring section 112.
In the bottom of the combustion furnace 113, fluidized material such as sand and fuel such as coal and sludge supplied from the fuel input port 110 are mixed and fluidized by air supplied from the first air supply port provided at the bottom to form a bed section 106 as a fluidized bed. Thus, combustion is carried out in the bed section 106. The temperature of the bed section 106 is controlled by flowing water or steam to the heat transfer pipe 111 provided in the bed section 106. Also, the convectional heat transferring section 112 is provided in the free board B108 as a combustion region above the bed section 106 to collect thermal energy of the exhaust gas by flowing water or steam in the convectional heat transferring section 112. For purposes of suppression of the generation of NOx and CO, the second air is supplied from the second air supply port 102. Generally, the bed section 106 is operated in the condition that an air rate of the first air quantity to a theoretical air quantity is 1.0 for the suppression of the generation of CO. The reason is as follows. That is, the temperature of a free board section A 107 is as low as 500 to 700xc2x0 C. because the combustion in the fluidized bed is carried out at the temperature of 800 to 900xc2x0 C. and the second air supply port 102 is provided above the bed section 106. When the fuel is combusted in the air rate of 1.0 or below in the bed section 106, a lot of CO is generated. The complete combustion cannot be carried out even if the second air is supplied. As a result, a part of CO is exhausted from the furnace output port 105. Therefore, in the actual operation, the air rate of the first air quantity to theoretical air quantity in the bed section 106 can be reduced only to about 1.0. For this reason, the bed section 106 is not set to deoxidation atmosphere, so that the generation quantity of NOx increases (150-250 ppm (O2 6% conversion)).
It should be noted that the cyclone 117 collects non-combusted ash in the exhaust gas. The hopper 118 stores the non-combusted ash. The stored the non-combusted ash is supplied to the bottom of the combustion furnace 113 as the fuel.
As described above, with the generation of the exhaust gas at the time of the combustion, it is not easy to achieve both of the suppression of generation of NOx and the suppression of generation of CO and dioxine kind at the same time. For the suppression of generation of NOx, it is necessary to realize a deoxidation atmosphere by decreasing an air surplus rate of a quantity of air supplied actually in the combustion to a quantity of air to be supplied for the complete combustion of fuel (theoretical air quantity). On the other hand, for the suppression of generation of CO and dioxine, it is necessary to realize an oxidation atmosphere by increasing the air surplus rate. That is, it is difficult to simultaneously suppress the generation of NOx, and the generation of CO and dioxine kind because of difference air surplus rates.
Therefore, an object of the present invention is to provide a fluidized bed incinerator and a combustion method in which the generation of NOx, CO, and dioxine can be suppressed at the same time.
In an aspect of the present invention, a fluidized bed incinerator having a combustion furnace includes first to fourth combustion sections. Fuel is supplied to the first combustion section and a combustion exhaust gas is exhausted after the fourth combustion section. First to fourth airs are supplied to the first to fourth combustion sections in first to fourth air surplus rates, respectively. The second air surplus rate is equal to or more than the first air surplus rate, the third air surplus rate is equal to or more than the second air surplus rate, and the fourth air surplus rate is equal to or more than the third air surplus rate.
Here, it is desirable that the first combustion section combusts the fuel in a first temperature range in deoxidation atmosphere by the first air, to suppress generation of NOx and dioxine. It is desirable that the second combustion section combusts a non-combusted component of the fuel in a second temperature range in the deoxidation atmosphere by the second air, to suppress the generation of NOx and dioxine and to dissolve NOx and dioxine generated in the first combustion section. It is desirable that the third combustion section combusts a non-combusted component of the fuel in a third temperature range by the third air, to suppress the generation of NOx and dioxine and to dissolve NOx and dioxine generated in the second combustion section, and a fourth combustion section carries out complete combustion of a non-combusted component of the fuel in a fourth temperature range in oxidization atmosphere by the fourth air, to suppress the generation of NOx and dioxine and to dissolve NOx and dioxine generated in the third combustion section. In this case, the first to third temperature ranges may be substantially the same, and may be a range of 800xc2x0 C. to 900xc2x0 C.
Also, the fourth temperature range may be equal to or lower than each of the first to third temperature range, and may be a range of 750xc2x0 C. to 850xc2x0 C.
Also, the first temperature range of the first combustion section may be controlled by a first temperature control section, and the fourth temperature range of the fourth combustion section may be controlled by a second temperature control section. On the other hand, the second and third temperature ranges of the second and third combustion sections may be controlled by changing the second and third air surplus rates, respectively.
Also, it is desirable that the first air surplus rate is in a range of 0.5 to 0.7, the second air surplus rate is in a range of 0.7 to 0.9, the third air surplus rate is in a range of 0.9 to 1.15, and the fourth air surplus rate is in a range of 1.15 to 1.6.
Also, a residence time of a combustion gas in the first combustion section is desirably in a range of 1.5 to 2.5 seconds, and a residence time of a combustion gas in the second combustion section is desirably in a range of 0.5 to 1.5 seconds. Also, a residence time of a combustion gas in the first combustion section is desirably in a range of 0.1 to 1.0 second, and a residence time of a combustion gas in the first combustion section is desirably in a range of 1.5 to 2.5 seconds.
Also, the first combustion section may be a fluidized bed combustion section, and have a first air supply port provided in a bottom of the first combustion section.
Also, the second combustion section may have a second air supply port is provided in a range of 1500 to 2100 mm from the bottom, the third combustion section may have a third air supply port provided in a range of 3100 3700 mm from the bottom, and the fourth combustion section may have a fourth air supply port provided in a range of 4100 to 4700 mm from the bottom. In this case, the fluidized bed incinerator may further include a fuel supply port provided between the second air supply port and the third air supply port.
In another aspect of the present invention, a combustion method in a fluidized bed incinerator is achieved by (a) supplying fuel to a fist combustion section as a fluidized bet; by (b) combusting the fuel in a first temperature range by first air supplied to the first combustion section, while suppressing generation of NOx and dioxine; by (c) combusting a non-combusted component of the fuel in a second temperature range by second air supplied to a second combustion section, while suppressing the generation of NOx and dioxine and dissolving NOx and dioxine generated in the first combustion section; by (d) combusting a non-combusted component of the fuel in a third temperature range by third air supplied to a third combustion section, while suppressing the generation of NOx and dioxine and dissolving NOx and dioxine generated in the second combustion section; and by (e) carrying out complete combustion of a non-combusted component of the fuel in a fourth temperature range by fourth air supplied to a fourth combustion section, while suppressing the generation of NOx and dioxine and dissolving NOx and dioxine generated in the third combustion section.
In this case, the (b) and (c) steps may be carried out in a deoxidation atmosphere, and the (e) step may be carried out in an oxidation atmosphere.
Also, the first to fourth airs are supplied to the first to fourth combustion sections in first to fourth air surplus rates, respectively. At this time, it is desirable that the second air surplus rate is equal to or more than the first air surplus rate, the third air surplus rate is equal to or more than the second air surplus rate, and the fourth air surplus rate is equal to or more than the third air surplus rate.
Also, the first to third temperature ranges may be a range of 800xc2x0 C. to 900xc2x0 C., and the fourth temperature range may be equal to or lower than each of the first to third temperature ranges, and a range of 750xc2x0 C. to 850xc2x0 C.
Also, it is desirable that a residence time of a combustion gas in the first combustion section is in a range of 1.5 to 2.5 seconds, a residence time of a combustion gas in the second combustion section is in a range of 0.5 to 1.5 seconds, a residence time of a combustion gas in the first combustion section is in a range of 0.1 to 1.0 second, and a residence time of a combustion gas in the first combustion section is in a range of 1.5 to 2.5 seconds.
In still another aspect of the present invention, a fluidized bed incinerator having a combustion furnace includes first to fourth combustion sections. The fuel is supplied to the first combustion section and a combustion exhaust gas is exhausted after the fourth combustion section. First to fourth airs are supplied to the first to fourth combustion sections in first to fourth air surplus rates, respectively. It is desirable that the residence time of gas corresponding to the fuel in the first combustion section is in a range of 1.5 to 2.5 seconds; a residence time of the gas in the second combustion section is in a range of 0.5 to 1.5 seconds; a residence time of the gas in the third combustion section is in a range of 0.1 to 1.0 seconds; and a residence time of the gas in the fourth combustion section is equal to or more than 1.5 to 2.5 seconds.
In yet still another aspect of the present invention, a fluidized bed incinerator having a combustion furnace includes first to fourth combustion sections. The fuel is supplied to the first combustion section as a fluidized bed section and a combustion exhaust gas is exhausted after the fourth combustion section. First to fourth airs are supplied from first to fourth air supply ports to the first to fourth combustion sections, respectively. It is desirable that the first air supply port is provided in a bottom of the combustion furnace, the second air supply port is provided in a range of 1500 to 2100 mm from the bottom; the third air supply port is provided in a range of 3100 3700 mm from the bottom; and the fourth air supply port is provided in a range of 4100 to 4700 mm from the bottom.
In this case, it is desirable that the combustion furnace further may include a fuel input port provided in a range of 2100 to 2700 mm from the bottom.